The Hebrew calendar (also referred to as the Jewish calendar) is based of the lunar cycle and governs many aspects of Jewish religious life, including the occurrences of religious holidays, observance of the anniversary of a death, etc. It has been used over the centuries by Jews throughout the world for such purposes. As a lunar calendar it is quite different and substantially more complex than the standard Gregorian calendar; however, the end result is that its months track the cycles of the moon with surprising accuracy over many millenia and maintain a proper relationship to the solar year and its seasons. Further, it is set up to prohibit the occurence of major holidays on certain days of the week. A relatively fixed relationship with the Gregorian calander is maintained at the end of each 19 year cycle. Study of this calandar and its intricacies has presented a challenge to Hebraic scholars and mathematicians alike, and as such poses some interesting problems for implementation into an electronic device. To assist in the explanation of the salient features of this calendar and the invention, several tables which can be found in the detailed description and specification have been included.
The Hebrew calendar consists of 12 regular months listed in Table I and a leap month which occurs only on the 3rd, 6th, 8th, 11th, 14th, 17th and 19th years of the basic 19 year cycle. In addition, 3 other months can vary in their number of days (between 29 and 30) as shown in Table II. All the remaining months are fixed at either 29 or 30 days in an alternating pattern. Careful study of the result of the regular and irregular yearly variations reveals that only six basic year types are possible, containing 353, 354, 355, 383, 384 or 385 days. These year types are indicated for the years 5736-5771 (1975-2011) in Table III. To establish a given year type, one must specify whether the year in question is a leap year and the number of days in the three months whose length fluctuates from year to year. Further analysis shows one of these months (Adar I) to contain 30 days only when there is a leap year and the leap month is added. The remaining 2 months, Heshvan and Kislev (shown in Table II) both vary between 29 and 30 days, but any true pattern in this variation would be so long that it cannot be seen in a period of as much as 200 years. (An almost exact cycle occurs every 247 (13.times.19) years, but this too is not precise.) This is because of a number of complex rules governing which day of the week the next new year must fall and the precise time of day for the new moon of the new year. The details of these rules will not be discussed here.
The minor irregular variations in the length of Heshvan and Kislev make an electronic implementation of this calendar based on a simple cyclical system virtually impossible. Further, the implementation described herein represents a significant advantage of cost and compactness over a computer system imcorporating the complex rules mentioned above. The present invention is a novel means for automatically tracking and displaying the Hebrew (Jewish) calendar date for a 10 year period in view of the intricacies described and in view of simplicity and reasonable cost. The salient features of this invention may also be applied to other calendars where there are complex or non-cyclical irregularities.
The invention shown herein can be used directly to display the Hebrew date and day of the week, or in conjunction with an electronic Gregorian calendar device to compute corresponding calendar dates for the two calendars. It can also be used to compute the day of the week for a specified Hebrew date. These applications are illustrated in FIG. 3. Because of the extreme accuracy with which the Hebrew calendar tracks the lunar cycle it could also be used to display the phases of the moon. Electronic and mechanical means have been developed to track the relatively minor variations in the secular (Gregorian) calendar and to display the date; however, there appears to have been no attempt to implement an electronic device to enable the automatic display of the Hebrew date from this calendar. Further, the use of prewired memory cards and appropriate gating enable the device to track irregular calendar variations over a ten year period. This is a novel approach which facilitates inexpensive conversion of the calendar for a new 10-year period, while minimizing the cost and complexity of the device.